Conventionally, there have been liquid applicators having a liquid pressing mechanism or liquid pressing means for pressurizing the application liquid inside the main body in order to supply the applying part with the application liquid in a timely manner.
For example, there is a proposal (see Patent document 1) of a liquid applicator that includes: an applying member attached at the front end of its barrel cylinder; a tank arranged inside the barrel cylinder for reserving a liquid in the rear of the applying member; a conduit hole portion for leading the liquid ejected from the tank to the applying member; and a liquid pushing means (liquid pressing mechanism) for pushing out the liquid inside the aforementioned tank to the applying member through the conduit hole portion by advancing a piston that is fitted in the tank so as to be slidable in its axial direction.
Also there is a proposal (see Patent document 2) for a liquid container including a main body having a tank portion holding a liquid therein; a feed mechanism having a front-end feeder joined at the front end of the main body for feeding the liquid; and an actuating mechanism for pushing the liquid inside the tank portion toward the feed mechanism.
Since in the liquid applicators as above, their brush-like applying member after usage holds a considerable amount of application liquid therein and is exposed to the outside air, there is the problem that the application liquid contained in the applying member degrades with time and is liable to be spoilt.
There is also a known configuration of a liquid applicator or liquid container having a mechanism for preventing against degradation of the application liquid with the passage of time or entrance of the outside air by using an elastic body for its applying part.
For example, there is a proposal of an application container which includes: an application container body for storing an application liquid therein; and an applying member disposed at the front end of the application container body, for applying the application liquid over an applied object and is used to apply the application liquid to the soft applied object by bringing the front end of the applying member into contact with the applied object, wherein the applying member is made of an approximately tubular elastic body with its front and rear ends open all the time and leads the application liquid in the application container body and ejects it from the front-end opening (e.g., see Patent document 3).
However, these liquid applicators are suitable for dribbling the application liquid but have the problem that it is difficult to apply the application liquid over a wide area in a simple manner. Also, there is another problem that they present difficulties when used in combination with a high viscosity application liquid because of structural reasons of their containers.
As described above, conventionally there have been liquid applicators for applying application liquids such as cosmetics, medial fluid and the like, which store an application liquid inside the main body and includes a liquid pressurizing mechanism or liquid pressing means provided with a piston and its advancing mechanism for pressurizing the application liquid inside the liquid storing chamber provided inside the main body in order to timely feed the application liquid to the applying member (applying part) arranged at the front end (see Japanese Patent No. 3081834: patent document 4, Japanese Utility Model Registration No. 2603088: patent document 5, and the like).
With such liquid applicators, drawing back the liquid that has been ejected to the applying part was previously considered to be problematic because the liquid that has come into contact with unwanted bacteria attached to skin and the like may be returned into the container.
Recently, however, owing to improvement in liquid's antibacterial power, this problem has become less important while there has been an increased demand for a liquid-restoring function to deal with the cases where the liquid has been excessively delivered, such as when the liquid happened to be ejected after a time lag when it was ejected by the piston, when the liquid expanded due to change in temperature, when the user made a pushing action more than needed and other cases.
Also, with a configuration where a fixed amount of liquid is ejected by pushing a piston while feeling a clicking sensation as in the cases of patent document 4 and patent document 5, no problem occurs when the advancing mechanism is operated in the direction of ejection. However, when the piston is retracted, there is a fear that unexpected deformation, breakdown and other damage of the mechanism occur as a result of application of a load more than required to a piston or shaft-shaped member.
FIG. 1 shows a piston body a that moves forwards and rearwards inside a conventional main body (inside a liquid storing chamber) and a front end part b1 of a shaft-shaped member b that engages boss a1 of the piston body, as is shown in patent document 4.
As sectionally shown in FIG. 1(a), cylindrical boss a1 having a space a2 opening rearwards, is extended at the rear side of piston body a. Formed in the inner wall of boss a1 that defines this space a2 is an annular rib c that is formed around the circumference and projected towards the center of the axis.
On the other hand, shaft-shaped member b is formed of a main body formed with a threaded portion b2 having a helical groove on its surface and a front end part b1 having an extended approximately cylindrical shape with no helical groove with a large-diametric flange portion d formed at its front end.
As shown in FIG. 1(b), front end part b1 of shaft-shaped member b is inserted into space a2 of the aforementioned boss a1 and further press-fitted thereinto, so as to make flange portion d climb over the aforementioned annular rib c, whereby front end part b1 of shaft-shaped member b is tightly fitted into space a2.
Conventionally, when piston body a is advanced, boss a1 in the rear part of piston body a is pushed forward by shaft-shaped member b, so that front end part b1 of shaft-shaped member b will not come off boss a1 of piston body a.
On the other hand, when piston body a is drawn back, the piston is pulled rearward by the engaging force of boss a1 in the rear part of piston body a with front end part b1 of shaft-shaped member b. This engaging force is brought about by flange portion d in the front end part b1 of shaft-shaped member b hooking annular rib c of boss a1, and mainly depends on the deformation-resisting force of boss a1.
When the application liquid is a high-viscosity fluid or a fluid whose viscosity has changed to be high due to environmental conditions such as temperature etc., there occurs a situation in which a strong force beyond the aforementioned engaging force needs to be applied to draw the piston body a.
In such a case, boss a1 becomes enlarged or deformed in any other way, so that annular rib c of boss a1 climbs over flange portion d of shaft-shaped member b, and shaft-shaped member b comes off boss a1, hence piston body a does not move, causing malfunction or unusability.
In contrast, if, in order to enhance the engaging force, the outside diameter of the aforementioned flange portion and the inside diameter of the annular rib are made greater so as to make the height of the step therebetween greater, not only the engaging force of the shaft-shaped member with the boss but also, the press-fitting force when front end part b1 of shaft-shaped member b is inserted and squeezed into space a2 of boss a1, naturally becomes greater. This increases the work load during manufacturing and also causes the problem of increasing the manufacturing time and manufacturing cost.
Further, there is a fear that with the piston body of a resin molding, the boss and the front end of the threaded portion deform more than required at the time of press-fitting, causing breakdown and cracks, inviting degradation of yield.
Patent document 1:
    Japanese Patent Application Laid-open Hei 9-322819Patent document 2:    Japanese Patent Application Laid-open 2004-89592Patent document 3:    Japanese Patent Application Laid-open Hei 9-192581Patent document 4:    Japanese Patent No. 3081834Patent document 5:    Japanese Utility Model Registration No. 2603088